Gas tubrine aircraft engines are generally adapted to provide bi-directional thrust: rearward thrust for propulsion and forward thrust for braking.
FIGS. 1 and 3 (FIG. 3 being a partial side view of FIG. 1) schematically illustrate an engine assembly 1 used for generation of braking thrust. An annular cowling 2 and a nacelle 6 surround an engine, the fan 2A of which only is exposed. The annular cowling 2 can be translated generally parallel with the engine axis 4 to thereby open a passage 5 between the cowling 2 and the engine nacelle 6. Simultaneously, blocker doors 7 (not shown in FIG. 1) are rotated into the path of the rearward flowing propulsion gases 7A in FIG. 3 to divert these gases into the forward direction as jets 8 in FIGS. 1 and 3 to provide braking thrust for the aircraft.
The forces necessary to achieve translation of the cowl 2 are generally applied by threaded rod-and-ballscrew assemblies (termed actuator rods) 10A-C in FIG. 1 and 10C in FIGS. 2 and 3 (which are a schematic cross-sectional view of FIG. 1 taken along the lines 2--2). The actuator rods are aligned generally parallel with the engine's axis 4. There are similar actuator rods on the side of the engine facing away from the reader in FIG. 1 to provide balanced forces to the cowl 2 but which are not shown. The actuator rods are attached to respective brackets, only one of which is shown in FIG. 1, and that one is designated by the numeral 12, and if fastened to the cowling 2.
The demands placed upon the brackets 12 are rigorous. First, the brackets are commonly required to withstand forces of 6500 lb. each which are applied by the actuator rods. A significant problem is encountered in transmitting such forces to the cowling 2 when the cowling 2 is contructed of a honeycomb material sandwiched between graphite-epoxy skins, partly because the strength of the adhesion of the skin to the honeycomb material is generally much less than the strength of the skin itself. That is, if the bracket is attached to one skin, the force transmitted by the bracket can break the adhesive bond between the skin and the honeycomb.
Second, a clearance problem restricts the amount of space available to house the brackets 12. As shown in FIG. 2, the cowling 2 has inner and outer parts 14A and 14B. As stated above, this cowling 2 slides left- and rightward as shown by arrows 15C. A fixed, nonmoving structure 14D is present between the inner and outer parts 14A and 14B, and the distance or clearance 22 can be quite small, of the order of 0.10 inches (about 2.5 mm). The brackets 12 must allow this clearance 22 during translation of the cowling 2 in order to avoid damage.
Further, the cowling 2 is generally the shape of a truncated cone: the downstream portion 26 is of smaller diameter than the upstream portion 28. Thus, since the actuator rods such as 10C are generally parallel with the engine axis 14, the clearance afforded them becomes progressively less as they extend downstream. This is shown in FIG. 2, wherein a side view of the cowling 2 shows that the clearance 30 between the fixed member 14D and the cowling 2 diminishes in the downstream regions of the actuator rod 10C. The brackets 12 are frequently located in a downstream region 30A having low clearance.
Third, the movement of the cowling 2 with respect to the nacelle 6 occurs over a short time interval: about 2 seconds. Thus, a high impulse load, as well as a high absolute load, must be borne by the brackets 12.